Because fuel costs represent a significant percentage of the overall cost of transporting cargo via a transport vehicle (e.g., tractor-trailers, semi-trailers, semis, over-the-road trucks, delivery trucks, straight trucks, box trucks, large good vehicles, railcars), businesses have an interest in reducing the amount of fuel consumed by their transport vehicles. It is well known that transport vehicles in motion experience a significant amount of aerodynamic drag (i.e., drag). This aerodynamic drag greatly reduces fuel efficiency, leads to increased fuel consumption, and increases the cost of transporting items.
When considering methods of reducing aerodynamic drag on a transport vehicle, the various effects of crosswind are of particular importance. When the movement of a transport vehicle is at a velocity significantly greater than the wind speed, air strikes the transport vehicle from the front. Crosswind conditions exist when a significant wind force is exerted on the transport vehicle from a direction other than the transport vehicle's line of travel. Such crosswind typically attacks the transport vehicle at an angle between about 1 degree and 12 degrees (e.g., about 6 degrees) relative to the direction of travel of the transport vehicle.
Crosswind has a significantly greater effect on transport vehicles in motion than it has on smaller vehicles (e.g., passenger automobiles). One reason that crosswind influences transport vehicles more is that such vehicles typically have a substantially larger gap between their front wheel assembly and their rear wheel assembly than do smaller vehicles. This large gap provides more space for crosswind to pass beneath the transport vehicle (i.e., in the space between the front and rear wheel assemblies, and between the bottom of the transport vehicle and the supporting surface (e.g., the road)).
As a transport vehicle moves through crosswind, an area of high pressure forms on the upwind side of the transport vehicle, and an area of low pressure forms on the downwind side of the transport vehicle. The airflow moves from the upwind area of high pressure, underneath and between the wheel assemblies of the transport vehicle, and then into the downwind area of low pressure on the opposite side of the transport vehicle. As the airflow enters the area of low pressure on the opposite side of the transport vehicle, it become turbulent, creates drag and reduces fuel efficiency. Turbulent air can also contribute to an increase in road spray (i.e., water or snow sprayed into the air by the wheels). Road spray can be annoying or dangerous to surrounding motorists because it tends to interfere with visibility.
Previous attempts to deal with airflow passing underneath a transport vehicle have focused on either preventing air from passing underneath the transport vehicle altogether, or deflecting air around the rear wheel assembly. There are various types of aerodynamic add-on devices employed for these purposes to varying degrees of success. Some examples of aerodynamic add-on devices include side skirts, end skirts, trailer forebody and aftbody plates, side enclosures, side extenders, vertical plates and air deflectors used at the tractor-trailer gap, belly boxes, air dams and other under-mounted or underside air management structures. Such aerodynamic add-on devices are often positioned on the bottom (i.e., undercarriage, underbody) of the transport vehicle and immediately forward of the rear wheel assembly in an effort to reduce the significant drag created by the rear wheel assembly. Such devices are generally effective at reducing the aerodynamic drag.
But to be truly effective, the aerodynamic device must account for the positive and negative influences on the transport vehicle of crosswind. A positive aspect of this crosswind is that as the air rushes underneath the transport vehicle from the side, it creates a low pressure area underneath the transport vehicle. This area of low pressure underneath the transport vehicle is desirable because it generates downforce on the transport vehicle. Greater downforce generally creates greater stability. A negative aspect of this crosswind airflow is that when the airflow reaches the area of low pressure on the downwind side of the transport vehicle, it becomes turbulent and creates aerodynamic drag. The turbulence also can cause the transport vehicle to become unstable due to buffeting. This instability can be both unsafe and fatiguing for the operator of the transport vehicle.
Some of the aforementioned aerodynamic add-on devices do not address the unique problems associated with the aerodynamic forces that crosswind exerts on a moving transport vehicle. Others have recognized the impact of crosswind, but have sought to address the issue by inhibiting the lateral flow of crosswind underneath the transport vehicle (e.g., by the use of skirts (i.e., trailer side skirts, side fairings)). The design characteristics of some devices may work in non-crosswind conditions, but those same design characteristics may actually contribute to aerodynamic drag and instability in crosswind. By inhibiting the lateral flow of crosswind beneath the transport vehicle, these devices potentially decrease stability and increase tipping forces (e.g., forces caused by crosswind having a tendency to tip over the transport vehicle), thereby resulting in danger to the operator of the transport vehicle as well as nearby motorists.
Furthermore, many transport vehicles are equipped with a rear bumper bar (e.g., a rear safety bumper (i.e., an ICC bumper). An ICC bumper, for example, is a rear bumper bar typically made out of 3-inch to 4-inch steel channel stock, suspended about half the distance from the bottom of the transport vehicle to the supporting surface. Although the rear bumper bar serves an important function of preventing underride collisions, it also creates aerodynamic drag by interfering with the smooth, organized (i.e., laminar) flow of air underneath and around the transport vehicle. The resulting aerodynamic drag decreases the fuel efficiency of the transport vehicle. In addition, existing rear bumper bars do nothing to help mitigate the drag resulting from turbulent airflow at the rear portion of a transport vehicle.
As discussed, a variety of aerodynamic add-on device have been developed to improve the aerodynamics of transport vehicles. Many of these devices are bulky and difficult for just one person to install. Installation can be further complicated by the fact that certain materials used to make these aerodynamic devices can contract and expand quite significantly in extreme temperatures. The installation methods currently employed (e.g., bolting or riveting the aerodynamic device in place) generally inhibit the ability of the aerodynamic device to expand and/or contract. As a result, such installation methods necessitate the use of materials which tend to not expand or contract, which materials are generally more expensive and heavier than alternative materials.
Accordingly, there exists a need for an improved apparatus capable of reducing aerodynamic drag resulting from a transport vehicle moving in crosswind, while increasing the stability and handling of the transport vehicle and reducing road spray. Furthermore, there exists a need for an apparatus that promotes the conversion of turbulent airflow to laminar airflow at the rear portion of a transport vehicle. There also exists a need for a system and method for installing an aerodynamic add-on device that is easier for one person to perform, and that allows for the expansion and contraction of the aerodynamic device in extreme temperatures, thereby permitting the use of lighter and less expensive materials in the manufacture of the aerodynamic device.